Components such as flaps on aircraft are often driven for movement by ball-screw actuators. Forces on the component, e.g. from fast-moving air on the flap, may feed back through the nut of the ball-screw actuator and produce axial forces and torque on the screw shaft. These feedback forces tend to bias one direction of operation over the other. A no-back device is commonly used to provide resistive angular force to counteract this feedback torque and substantially balance the operational torque required by the motor to actuate the component.
It is desirable that the no-back device provide little resistance to torque originating from the driving motor, so that the torque is freely transmitted to the screw-shaft and can act to move the component in either direction, regardless of the direction of feedback torque.
U.S. Pat. No. 6,109,415 provides a prior-art no-back device in which a section of the ballscrew actuator shaft is received in a housing that allows very limited axial movement of the shaft. When the shaft is under compression, it experiences feedback torque in a first direction because the axial load exerted on the nut forces the balls of the nut in an axial direction against the inclined pitch of the screw thread on the shaft, urging the shaft to rotate and generating torque in the shaft. The load on the shaft also urges a flange of the shaft against a resistance wheel which in turn is pressed against a ratchet wheel. However, the ratchet wheel is orientated so that rotation is blocked in the direction of the feedback torque (the “feedback” direction). As a result, the resistance wheel generates a resistive force via friction with the flange and the ratchet wheel which acts counter to the feedback torque being transmitted by the shaft via the flange. In this way, the tendency of the nut to want to move along the screw thread is counteracted by this resistive angular force. The motor can then drive the shaft in different directions using substantially equal amounts of force.
In U.S. Pat. No. 6,109,415, the resistance wheel comprises a series of rollers that are arranged in the plane of the resistance wheel but inclined to the radial direction. In the feedback direction, when the rollers are providing a resistive angular force, the relative angular movement causes the rollers to slide against the surfaces of the flange and ratchet wheel to generate the angular resistive force.
The no-back device of U.S. Pat. No. 6,109,415 includes a second ratchet assembly and a second braking device arranged on the opposite side of the flange, so that when the shaft is under tension and experiencing feedback torque in an opposite rotational direction, the flange engages with a second resistance wheel which is in turn urged against a second ratchet wheel. The second ratchet wheel is arranged to block rotation in that opposite direction, causing the resistance wheel to generate a resistive angular force reacting to the feedback torque.
Such conventional no-back devices have generally been considered satisfactory for their intended purpose. However, it has been recognized that the braking forces generated through surfaces rubbing against one another can cause debris to accumulate within the no-back device. Such foreign objects can cause damage to the internal working of the no-back device and may compromise its reliability. There is a need to provide a no-back device which runs a lower risk of generating foreign objects (e.g. scobs) within the mechanism.